The invention relates to a flat-track unit for motor-vehicle test beds comprising a roadway-simulating looped belt which is guided over two rollers and accommodates a vehicle wheel on its top upper test side which is orientated roughly horizontally at the ground level. The two rollers are mounted in a frame which is supported in a vertically floating manner and is mechanically guided in the vertical direction by a spring mounting which absorbs the proportionate dead weight of the flat-track unit and the wheel load.
A rectilinear supporting shoe located between the rollers supports the test side of the belt in a guiding or rolling manner at the contact region with a vehicle wheel. The supporting shoe is vertically movable relative to the frame by a vertically orientated hydraulic shoe excitation cylinder which acts centrally on it. The cylinder can be excited to develop vertical oscillations for simulating roadway vibrations. One of the rollers is connected via a universal-joint shaft to a drive and braking unit for simulating traction and braking forces. The device and braking unit is arranged so as to be roughly on the same axis as and removed from the roller in a vertically immovable position next to the frame.
U.S. Pat. No. 3,520,180 shows this general type of flat-track.
Flat-track units of this type are intended to reproduce on the test bed, the rolling characteristics of the vehicle wheel on the road in as true to nature manner as is possible. In the process, the vehicle wheel stands on a flat contact surface which can be driven or even braked to simulate braking or traction forces. In addition, the roadway vibrations are simulated by vertical oscillations being imparted to the flat-track unit. Since the belt cannot itself provide a flat contact surface on account of its flexibility, the belt is supported at the contact region of the vehicle wheel by a supporting shoe which is flat on its top. This supporting shoe is excited by a vertically disposed excitation cylinder to simulate the roadway vibrations. In the previously known flat-track unit, the frame carrying the rollers is supported elastically on the supporting shoes and the latter, by a vertically disposed excitation cylinder, is excited to develop vertical oscillations. In order to avoid resonances of the frame relative to the supporting shoe, the frame is restrained on a foundation via oscillation dampers. So that the frame can be of particularly light construction, the power source for the roller driver is arranged in a fixed position removed from the frame, via movable universal-joint shafting. In this known flat-track unit, it is disadvantageous that the supporting shoe can accomplish large unchecked amplitudes of oscillation relative to the frame. Since these amplitudes of oscillation cannot readily be bridged by the belt, this requires that the belt be overstretched. Furthermore, it is disadvantageous since the excitation energy has to be very high on account of the damping of the frame oscillation. This results in heat in the oscillation dampers due to energy loss and this heat unfortunately has to be discharged at this location.
The object of the instant invention is to further develop the above-referenced flat-track so that the excitation energy necessary for a vertical excitation of the flat-track unit is reduced and that no large unchecked amplitudes of vertical oscillation of the belt relative to the frame can occur.
This object is achieved according to the invention by having the spring mounting of the frame rely on hydropneumatic springs which have a cylinder housing rigidly connected a least indirectly, to the foundation. A supporting shoe excitation cylinder arranged completely inside the belt loop is rigidly supported at its bottom to the frame. During vertical excitation this shoe excitation cylinder is limited to the short-stroke and high-frequency oscillation portions of the entire excited oscillations simulating the roadway vibrations. The frame is supported rigidly at least indirectly on the foundation by means Of a further hydraulic frame excitation cylinder acting rigidly at the bottom and roughly centrally on the frame. This further frame support cylinder can be excited to develop vertical oscillations which are limited to the long-stroke, low-frequency oscillation portions of the entire excited oscillations.
Owing to the separate and forced excitation of the supporting shoed with short-stroke and high-frequency oscillation portions and with the frame being excited with long-stroke and low-frequency oscillation portions, an unchecked oscillation of the supporting shoe relative to the frame is avoided. Nonetheless the long-stroke vertical oscillations that occur in reality are readily reproduced on the flat-track unit. Due to the fact that damping of oscillations is now no longer necessary, the excitation energy can be kept low.
It is advantageous if the frame has a cross piece, which passes through the belt loop for supporting the supporting-shoe excitation cylinder and a further cross piece is arranged below the belt loop connection to the piston rod of the frame excitation cylinder. The frame is essentially formed by two side cheeks arranged on both sides of the belt, and are connected by the two cross pieces to carry the bearings for the rollers. Two side cheeks, as viewed parallel to the roller axes are designed in rough approximation as a trapezoidal shape or extended V-shape.
The cylinder housing of the frame excitation cylinder is fixed, in terms of tilting and rotation, at least indirectly to the foundation. Its piston rod is designed as a guide column connected to the frame so as to be fixed in terms of tilting and rotation and is guided in a stable manner in the cylinder housing in such a way that the frame excitation cylinder also serves as a mechanical vertical guide for the frame. This piston rod extends axially past an upper and lower surface area and is guided over the entire longitudinal extent in the cylinder housing of the frame. The piston is designed as an annular collar encircling the piston rod with working spaces acting upon the annular piston in both directions.
It is also advantageous if the vertical spring mounting of the frame be in the form of a hydropneumatic spring having an annular piston and working space which can be acted upon hydraulically by pressure stored in a gas-pressure accumulator, which pressure remains largely constant irrespective of the stroke. This piston can be constructionally integrated with the frame excitation cylinder in such a way that the piston rod is provided with a further annular piston to which a hydraulically separate working space is allocated at the bottom.
The further piston is likewise designed as an annular collar encircling the piston rod which has a hollow inside to reduce weight.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.